Automatic measuring op cross taiik



March 20, 1945. E. P. FELCH, JR

AUTOMATIC MEASURING 0F GROSS TALK Original Filed April 3, 1940 4 Sheets-Sheet 1 A T TOR/VEV March 20, 1945. E. P. FELCH, JR

AUTOMATIC MEASURING 0F CROSS TALK Original Filed April 3, 1940 4 Sheets-Sheet 2 /M/E/v TOR E P. F ELCH, JR;

A Tron/95V Malh 20, 1945- E. P. FELcH. JR

AUTOMATIC MEASURING OF CROSS TALK Original Filed April 5, 1940 4 Sheets-Sheet 3 /NVE/v ron E'. P. FELCH, JR.

A T` TURA/Ey Map'h 20,1945. E p. FELCH, JR.l Re. 22,620

AUTOMATIC MEASURING 0F CROSS TALK Original Filed April 3, 1940 4 Sheets-Sheet 4 @wir {lllllll-lll SWEEP c/RcU/r E. P. F15/ cgi JR.

ATTORNEY Reissued Mar. 20, r1945 l l UNITED STATES PAT-ENT ori-lcs N AUTOMATIC RIESURING'OF CROSS TAL-K Edwin1P.;Felch.r.,10lmtham, N. J., .assigner to Bell .Telephone 'Laboraattores,-

Incorporated,

New lYork, N. Ywa corporation .of 4New Yori:4 Original No. '12354;601, dated September Af2., T941,

vSerial No. 327,570, April "3, 11940.' 'Application 'forreissue August 1.9-, 1942, Serial No. 455,353

127 claims. (ci. l1ra- 175.141) l l "This invention relates in certain speci-fic aspects to electrical measurements 'in intelligence transmission systems, and more vparticularly to ra 'method of andapparatus for expeditiously measuring cross-talk in open-Wire carrier'current sysltems.

Theinvention will -be claimed herein, and 'was 'claimed inthe original patent, as involving vfeatures which, although `disclosed ras embodied in 'a 'cross-talk measuring vsystem are not restricted to'such use. r

'The relation of pairs in an vopen-Wire 'line on the same row of poles brings the `carrier current circuits into 'such close proximity t0 each `other that some degree of cross-'talk exists therebetween. 4'I'he reduction of cross-talk to tolerableY levels Vvis Van important factor in the development of lbroad band carrier systems on open-'wire lines, and as cross-talk 'increases with frequency, Ii't is imperative to have accurate cross-talk data for such development. From experience with lower frequency systems, fundamental coupling coefficients for common open-wire congurations have been evaluated. Computation of the systematic components of cross-talk occurring with an ideal configuration as a .result of a change in the phase of cross-.talk between, transpositions constitutes a basis for determining the 'transposition of open-wire conductors embodied in `carrier current systems.

The existence of -dissymmetries `and the yextent .thereof .between open-wire lines kcan only 'be determined by making measurements on ac- I tual .lines in the field. To obtain .adequate cross- ,talk -data .so as to evaluate the random effects of .small variations from ideal open-wire configuray it would be necessary to make thousands of .measurements from which the effects y,of ysag diner-ences, elect, submarine sections and tree- Wire could be determined. Not only ywould suc-h measurements be helpful in vascertaining the accuracy-of calculated transpositions but they also would of considera-ble assistance in determining the feasibility 4of incorporating certain. openwire lines in broad .band carrier systems. vA realization of the magnitude' of such A`field program may `be had when .it is considered that in v marry cases `25'0 cross-talk frequency curves :are required for complete data on each repeater section of 60 to 100 miles in length. Point-by-point measurements are of doubtful value in 4ascertaining `,the trend of 'such curves unless measurements are made at a relatively large number of frequencies. Assuming 40 points to .be taken for veach frequencyrun, the number fof vmeasurerrients per section lwould be over 10,'000 'for-:a :101150 -150- kilocycle range -of measurements. .rl'lhis' Awould represent an extensive undertaking'from both labor and time standpoints. Accordingly, this 'invention contemplates high-speed automatic cross-talk measuring apparatus that would bo'th "substantially expedite` and simplify 'a study of the feasibility oflcertainopen-'wire lines for 'br'oad band carrier current systems Aand 'a'check up of the yaccuracy of computed transposit'ions.

Itis an object of `the' invention I'tovprov'ide rapparatus for expeditiously measuring cross-talk in carrier current systems. y

'Itis anotherobject of `the invention to provide va method of and 'apparatus for yautomatically "measuring cross-talkover a range of frequencie in broadband carrier current systems. p

Inaccordance `with a preferredem'bodiment of theinvention, alternating current energy whose frequency varies over a range within which crosstalk measurements .are to be made is applied to one end of a disturbing painfin an open-wire carrier current system ,and a. portion .of 'such energy ispassed as cross-,talk to a disturbed pair extending side by side wthvthe disturbing pair on the same v.rovv'ofvpoles 'The energy received yat the Opposite end4 of the disturbingpair is utilized to control theproduction of ,other alternating currentenergy varying over ai different lfrequency range and having a constant frequency =dierence therefrom. A portion of the other energyis heterodyned with the cross-talk re- -ceived at `the opposite yendof the disturbed Vpair to produce .-a certain heterodyned component. of constant- .frequency ythroughout the frequency range of cross-talk. This component. is preferably -demodulated to effect an .audible alternating current .component having a constant frequency and Whose amplitude variations represent cross-talk at each .frequency over the -frequency range of the: cross-talkto be measured.

Suitable apparatus responsive to such amplitude variations is employed for recording- -o`r indicating purposes.

`A feature of the invention is that both recording `and indicating are accomplished `automatically over `the range of alternating current; Waves applied to thepne end of the 'distlnbing pair. Another feature 'is that while "cross-talk has a diifer'ent frequency at each successive instantthe other alternating current waves to be .heteroldyned therewith also have` a ldifferent frequency at each successive instant but the frequency difference therebetween is constant. Still another feature is that the response to control vby the alternating current waves applied to the disturbing pair is practically instantaneous so that an immediate indication of cross-talk is secured regardless of the frequency thereof. Other features are a threshold arrangement to prevent response to spurious signals and a control of sensitivity so as to be most effective within a l narrow range of frequencies.

The preferred embodiment of the invention herein disclosed will be understood by reference to the following description taken together with the accompanying drawings, in which: I I

Fig. 1 illustrates schematically two pairs of conductors between which cross-talk is to be measured automatically in accordance with the invention;

Fig. 2 is identical with Fig. 1 except that it shows the invention in greater detail;

Fig.' 3 represents a sensitive arrangement for automatically controlling the output of an alternating current wave generator thereby tuning the receiving terminal over a range of measuring frequencies;

.Figs 4, 5, 6 and illustrate the operation of Figs. 1, 2 and 3;

Fig. 8 shows an alternate form of Fig.

Fig. 9 represents the action in Fig. 8.

Fig. y1 is a condensed schematic representation of the apparatus employed for automatically 3'; and

, measuring cross-talk passing from a disturbing pair to a disturbed pair both of which conductor pairs extend side by side in an open-wire line on the same row of poles between a sending terminal at the left and a receiving terminal at the right. The distance between theseterminals may be equivalent to a repeater section of 60 to 100 miles in length, which sections when joined together through their respective repeaters constitute a complete open-wire transmission line. Each conductor pair may accommodate a plurality of carrier channels depending on 'the frequency band utilized for each channel and the separation therebetween.

At the sending terminal an oscillator .I isl applied to the disturbing pair while the disturbed pair is terminated in a suitable network I 6. The vsending oscillator I5 is preferably a heterodyne type and is arranged to provide aconstant output over a desired range which in the present illustration is from 10 to 150 kilocycles within r0.5-decibel variation. Also, it is equipped with a motor drive so as to be driven synchronously through its range at a rate of 1/3, 1 or 3 kilocycles per second. Although not shown, the synchronousl drive embodies facilities for limiting the frequency range to the desired band, a'warble 'condenser for varying the output frequency over a 3-kilocyc1e band about the indicated lmean value at a rate of 6 complete cycles per second, and an intermittently actuated contact for providing frequency reference marks on record paper embodied in receiving apparatus in a manner that will be subsequently explained.

The receiving terminal of the disturbing pair is connected through a transformer I1 and a variable equalizer IB to an automatic frequency control apparatus I9 whose function will be presently described. Bridging the secondary winding of the transformer I1 is one sidev of a E30-decibel calibrating loss network whose opposite side is connected to one pair of end terminals 2i, 2l of a double-pole double-throw switch 22, whose other pair of end `terminals 23, 23 is applied through a transformer 24 to the receiving terminal of the disturbed pair, and whose center terminals 25; 25 are applied through another variable equalizer 26 to the input of a heterodyney detector 21. The latter is also connected to the automatic frequency control apparatus I9. The output of the detector 21 is supplied to a recorder 28'and a visual indicator 29 in parallel, either one orbcth of which may be used as preferred.

ALet it be assumed that initially far-end crosstalk vmeasurements are to be made. By definition, far-end cross-talk is the ratio of signaling energy appearing at the receiving terminal of the disturbed pair to that appearing at the receiving terminal of the disturbing pair, providing the signaling energy is applied to the remote end ofthe disturbing pair. As the oscillator I5 is arranged to supply the measuring waves at a constant level to the sending end of the disturbing pair, it will be apparent that such level will be reduced at the receiving terminal thereof, due

to the attenuation of the disturbing pair. Moreover, at the receiving end of the disturbing pair such level will not be constant, but will vary with frequency. n

As the detector 21 measures only absolute levels rather than ratios, it is first necessary to modify the at gain-frequency characteristic thereof to complement the loss-frequency characteristic of the disturbing circuit. This is accomplished by means of the adjustable equalizer 26 whosefunction is well 'understood and is described generally in' the patent of Zpbel, No. 1,603,305, issued October 19, 1926. 'I'his means that the level of the measuring waves supplied to the detector 21 at the receiving terminal via the disturbing pair, loss network 20 in its zero .positionand switch 22'in its right-hand position would be the same as if the equalizer 26 were omitted and the oscillator I5 were controlled to compensate for the loss-frequency characteristic yof the disturbing pair. In either case, the result is the same, that is, measuring waves of a ccnstant level would be supplied to the detector 2l. A similar purpose is fulfilled by variable equalvizer I8l disposed in the input of the automatic frequency control I9.

The recorder 28 and indicator 29 are calibrated in cross-talk units, and suchreadings are obtained by actuation of the calibrating loss network 20 in a manner that will now be explained. By definition, onecross-talk unit equals a decibel power ratio between adjacent disturbing andA disturbed pairs embodied in'an intelligence transmission system. In other words, this means that'for such circuits, having equal impedance, a 1/1,000`,000 part of the current in a disturbing circuit is transferred to a disturbed circuit. Therefore, the loss network 20 is initially actuated so that the entireV GO-decibel loss is inserted in the circuit, assuming the switch 22 is closed in the right-hand position. The gain of the detector 21 is then adjusted until a reading of 1,000 cross-talk units is produced on the visual indicator 29. Thereafter, the switch 22 is actuated to the left-hand position thereby removing the loss network 20 from the circuit and applying the disturbed pair through the variable ywith. the recorder 2li.

' frequency control 119i includes. an amplifier 95 olii auch energy passes as cross-talk intothe dis turbed pair and: is applied through the variable equalizer 26-v to the detectory 2.1whose output is divided between'the recorder 28. and:- indicatoi` 29. Thus, a visual representation of the cross'- talk passing from thev disturbing. to the disturbed pair is provided by the indicator 29 which. may

have a multiplier, not shown, associated therewith to provide readings on scales of l to. 10, 1'0 to 100; or' 100 tol 1,000. cross-talk A chart embodied in recorder 28 indicates changes in. attenuation required to maintain a constant level, that is, an attenuation-time curve;` This is readily translated into an attenuation-frequency curve by means of the synchronousmotor drive and an arrangement driven. thereby tov produce identifying marks preferably at. the f right-hand edge ofA a chart embodied in the recorder 28 in aimanner that will now be'described.

The arrangement for producing identifying marks onsuch chart is well known and briefly comprisesI a film driven by the synchronous motor associated. with the oscillator l5 so that at predetermined intervals a perforationzin the'film allows the closure `of an electrical contact. and thereby the completion of a discrete electrical circuit', not shown.. which extends. between .the sending and receiving terminals and embodies a solenoid and plunger both of which are associated energizes the solenoid: which actuatesthe plunger to mark the chart. Passage of the'film over the perforation serves to openy the electrical contact to cause a. deenergization of the solenoid which then permits the plunger to return toV its normal 'r position to await the next actuation. In this illustration, a single identifying mark is produced at each l-kilocycle point and. three successive marks| at the respective 50, 100 and 150 kilocycle points. A` lm arrangement that may be: modified to accomplishthe aboveis illustrated in the patent of T. Slonczewski, No. 2,058,641, issued; Octoberv 27, 1936.

`The automatic frequency control' I9' actuates the detectory 2l such that as the frequency:l of

Icross-talk supplied to the latter varies at each successive instant, the detector 21v supplies alter.- nating current waves having a predetermined constant frequency to the recorder 29 and indicator: 29. In other words, as cross-talk* varying from through '150 kilocycles is applied to. the input of detector 21, the automatic frequency control I9 is arranged to supply theretoy at the same time other alternating current waves varying in frequency between 475 and 61'5 kilocycles kilocycle cross-talk so that a continuous and instantaneous measurement of the latter may be ellfectedv in terms'k of variations in the amplitude ofthe 1kilocycle wave, either on the chart irr- 2 eluded in the recorder 28 or visually o'nthe indicator 29.

Fig. 2 shows in further detail the organization of. the automatic frequency control I9 and the detector 21. Thus it is seen that the automatic Completion of. suchcircuit embodying delayed automatic volume control and. Whosel output is supplied to a modulator 3.6 which may be of' a. suitable typen.` From the. outputtof the latter a predetermined modulation component. may be. selected by a filter 31' and applied through a. constant output amplifier 38 and a lter 319 to the input of a crystal frequency discriminator 4U: which controls, during certain intervals, the magnitude of a direct current volt.- age in response to changes in the frequency of the predetermined modulation component. This direct current voltage controls sweep circuit 4| which in4 turn actuates a. reverse feedback oscillater'` 4.2.' to produce the 475 to l'-ki'locycle range of other alternating current. waves in a. manner that will be presently explained. These latter waves are. amplified in isolating amplifier 43 whose function, in addition to amplication, is to preclude reaction ofthe circuits toy which its output: is app-lied upon the controlled oscillator 42.

One portion of the 475 to l-kilocycle. output ot' the amplifier4 43. is supplied lthrough amplifier 44'. to the modulator 3% to bev combined therein with the l0 to 150-kilocycle measuring'waves during an interval-when the latter are being received over the. disturbingv circuit whereby the predetermined modulation-component which, in this illustration, is. 465 kilocycles, is eiectedl Variations the frequency of thi-scomponent cause changes in. the magnitude. of the direct current voltage which controls the sweep.. circuit 4l and thereby the controlled.v oscillator 4-21 such that the heterodyning of; the porti-onY of the 475 to 615.kilocycle waves. and the 1'0 to 150-kilocycle measuring waves-in the modulator 38 tend-s to maintain the predetermined modulation component at the 465- v kilocycle frequency. During an interval of no input of lO to l50kilocycle measuring Waves to the amplifier 3"-5, the frequency discriminator 40 does not affect the direct current voltage and hence exerts.. no influence on the sweep circuit4'l'. Therefore, as.. it will be hereinafter pointed out, the latter merely serves to sweep the controlled oscillator 42 through its 475to 615kilocycle fre-1 pliier to: a' suitable modulator 46 embodied in detector 21. Also, amplifiers 44 and 45 serve the additional function 'of reducing cross-talk between the modulators 36 and 46 at the frequencies of the measuring waves and the several heterodyned components effected thereby. The detector 2T includes: amplifier 5U which is arranged with suitable networks, notshown, to reduce by negative feedback the response of the detector 2'!" to a'46'5-kilocycle component produced iu'a manner that A will be presently mentioned. In addition, amplifier 5l)v embodies a network, not shown, preferably to reduce the response of the detector 2T' tothe upper side-band components produced in the modulator 4E. Also, it is to be understood that amplifier 50 is 'provided with a fiat frequency-attenuation char.- acteri-stic. The amplified l0 to `150--kil'ocycle cross-talk in the output of amplier y5l! and the other portion ol" the amplified 4-75 to S15-kilo- 'heterodyned in the modulator 46. automatic self-tuning of cross-talk hereinbefore yinductance 11.

cycle waves in the output of the amp1i1ier45 are Thus, the

referred to is effected by the automatic frequency control I9 in response to the 10 to 150-kilocycle vmeasuring wavestransmitted on the disturbing band width selects a predeterminedv modulation component which in this illustration has a frequency of 465 kilocycles.v This component is impressed through an amplifier 52 on a demodulator 53 to be heterodyned therein with a 466- kilocycle alternating current wave furnished by an oscillator 54. From the output of the demodulator 53, a tuned amplifier 55 selects a 1-kilocycle heterodyned component which, after ampliflcation, is utilized either in recorder 28 or in amplifier-detector 51 and visual indicator 29, both of which operate essentially along the lines shown in the patent of F. E. Fairchild, No.

1,914,414, issued June 20, 1933.

The frequency discriminator 40, sweep circuit 4I and controlled oscillator 42 comprise a control arrangement to provide the waves extending from 475 to 615 kilocycles. Referring to Fig. 3, the frequency discriminator 4I includes input terminals B5 and 66 connecting a source of alternating current waves, not shown, through a relatively high resistance B1 to the input grid and cathode of tube VTI. A second grid is applied to biasing resistances 68 and 69. A piezo-crystal shunts the input grid and cathode. The positive terminal of a B battery supply is impressed through'a resistance 1I on the anode of VTI whose anode-cathode circuit is connected in shunt of a sweep capacity 12 embodied in sweep circuit 4I which also embodies a cold cathode gaseous discharge tube VT2. Across one control electrode and the anode of the tube VT2 is a capacity 13 whose function will be hereinafter explained.

Controlled oscillation .42 includes a control tube VT3 and a resonant network 14 including in parallel acapacity 'I5 and winding 16 of an The resonant network 14 is applied to the anode of the control tube VT3 and the screen of the tube W4 whichV screen functions as an anode. The winding 18 of the inductance 16 is connected to the control grid of the tube VT4. Disposed in the anode circuit of the tube VT4 is an inductance 19 which resonates with the circuit capacitance below the frequency of the oscillation of the circuit and hence exhibits the negative reactance characteristic of a capacitance while, at the same time, providing a direct current path in the anode current. The resonant network 14 is tuned initially tol a frequency which is slightly below 475 kilocycles, principally by circuit capacitance.y A resistance 80 in bridge of the winding 1E serves topprovide for the inductance 11 a Q of substantially 5. The anode of oscillator tube VT4 is directly connected to the screen of control tube VT3. The 475 to 6l5-kilocyc1e range of waves is taken oi the output electrodes of the oscillator tube VT4.

In the controlled oscillator -42 there are two parallel paths, a first path comprising the winding 18, control electrode and screen grid of the oscillator tube VT4, and the resonant network 14, and a second pathl embodying the anode of the oscillator tube VT4, screen of the lcontrol tube VT3, the resonant network 14 and the screen of the tube VT4. Consider'now two voltages impressed on the resonant network '14 via y14 via the hereinbefore-mentioned second path may be varied by changing the screen grid-anode ytran'sconductance of the control tube VT3. This is accomplished by adjusting the direct current bias impressed on the control grid thereof by varying the charge on the sweep capacity 12. As previously mentioned, the resonant network 14 will oscillate at that frequency at which the two voltages supplied thereto have zero phase shift therebetween. This may .be seen in Fig. 4 in which it will be observed that approximately at 64 volts applied to the -control grid of the control tube VT3, zero'phase shift between the two voltages supplied to the resonant network 14 via the aforedescribed-two parallel paths occurs at about 500 kilocycles; and also at l volts applied to the control grid of the control tube VT3, a similar condition obtains at about 600 kilocycles.

It will be understood that additional curves may be plotted in Fig. 4 to show the control grid potential of VT3 and the corresponding frequencies at which the two voltages supplied to the resonant network 14 have zero phase shift therebetween.- Therefore, it will be obvious that the constantsl of the resonant network 14 and the potentials applied to the control grid of the control tube VT3 may be arranged such that the resonant network 14 will oscillate over a desired range of frequencies. lustration a 60 to 90-volt range of` grid potential for the control tube VT3 will effect in the output of the tube VT4 a frequency range which extends from 475 to 615 kilocycles. This is shown in Eig. 5.

" Assuming no input is applied to the terminals 65 and 6E in Fig. 3, the sweeping capacity 12 is slowly charged through the anode resistance 1I from theB battery supply associated with the anode of tube VTI, until discharge is instituted in the gas tube VT2. Thereupon, the capacity 12 discharges rapidly through the low impedance of the discharged gaseous tube VT2 until the extinction Voltage thereof is attained, whereupon the gas tube VT2 will be returned to the non-conducting condition. Thus, the cycle of charging and discharging the capacity 12 may be repeated until interrupted in a manner that will be presently explained; During each cycle, the capacity 12 effects a Zero to 30volt-variation in the potential impressed on the control grid of the tube VT3. Referring to Fig. 5. itis seen that such voltage variation is adequate to sweep the controlled oscillator 42 over its frequency range of 4'75 to 615 kilocycles. The repetition of these `cycles may be referred to asthe huntingaction of the sweep' circuit 4I. The sweep circuit constants are such that each sweep cycle is approximately one-second duration which is For the purpose of this il.

ammo.,

determined by Ithe time required to charge. the capacity 12 to the breakdown voltage yoff `gaseous tube VTZ.

During the interval. of no input to the termi-k nails 65 and E6 in. Fig. 3, the screengrd of VTI v is normally biased by the voltage across the ad'- justable resistances 68 and B9 until the plateimpedance thereof is substantially 1 megohm. Consequently', the shunt path embodying. the anode-cathode circuit -yof VTI has practically no effect on the charging and discharging of the sweep condenser 12 and therefore no effect on the hunting action of the sweep circuit 4I. criterion for determining the adjustment of the resistances 68 and 69 is that the sweep circuit 4I should function once per second as stated hereinbefore. Too low a bias and hence too low a plate impedance tends to reduce the voltage of the sweeping capacity 72 below the breakdown voltage of the gaseous tube VT2 and' thereby to terminate the sweeping action whiletoo high a bias tends to increase the sweep rate beyond a.

range of utility'.

When an alternating current wave, say for example one having substantially a frequencyrv of 465 kilocycles, is applied across the terminals 65 and 66 in Fig. 3. and thereby across the piezo.- crystal 10,. such wave is also impressed on the input of the tubel In ythe latter this causes a rectification action in. theanode-ca'th'ode' cir-.- cuit. Such action serves to increase. the ow of space current and' toflower the impedance of the anode-cathode circuit.l This results in a correh` sponding decrease in the.v impedance of thea'nodecathodeA circuit shunting. the sweeping `capacity 12, as previously described. Now, the charging.v current supplied through the resistance II' from' `the B battery source is -divided between the anode-cathode circuit of thetub'e VTI and the sweeping capacity 12. Consequently, the voltage across the sweepingl capacity 12 is reduced to a..

value which is less than that required. to institute discharge in the gaseous tube VTZ.

Hence, the;

hunting action of the sweep circuit 4I is arrested and therefore the gaseous tube VT2 rests in. an undischarged state. Now, they voltageacross they sweeping capacity 12 applied to the control grid of the control tube VT3 is entirely dependent on the frequency of the alternating current waves applied across th'e terminals 65 and 66|.

The impedance of the piezocrystal 10 `between series andparallel resonance is. a. critical function (if-frequency. Assuming. the voltage impressed.

thereon is supplied by a constant voltage source,

such as the intermediate frequency amplifier 38,

STI-Fig 3, the voltage impressed acrossv the piezocrystal 10 is a critical function of. frequency.y

cuit of the tube VTI, occurring at the parallel resonance frequency of the piezocrystal 10, that is, at.465 kilocycles. .Positive and negative'varia'- tions in the 465kilocycle frequency are reflected as-'further impedance variations corresponding to-.certain voltage changes in the grid-cathode input ofthe tubefVTI. i

Fig. 2, in series with the scacco-ohm 'resistance 55' that a deviation of 45y cycles Aorr less. in. the -465e kilocycle modulatedv component. appliedacross tion in the. 4impedance of the anode-cathode cir'- cuit 'of VTI that the .variations in the Vcharge on the sweeping" `capacity 1121'changes the vbias in the. control of the. control: tubeVTS 'an amount that is adequate to sweep the output of the. oscillator' tubev VT4 over vits entire range. of 475 to 615 kilocycles. `The frequency discrim=.

inator 40 maintains the output of the oscillator tube VTI within 5I cyclesofthe proper frequency over the entire range of measuring frequencies, that` is, over 1-0' to 150 kilocycles. The control action is rapid enough to follow r1.5-ki'locycl'e wai-ble six. times: per second, Fig. 5'. shows the variations in the bias on the control grid of the tube VI'IB in response to: changes inthe testing frequency of 10 to 150` kilocycles to provide variations in thel output of the controlled oscillator 42. so that a.- modulated component; substantially havingia frequency of 465 kiiocycl'es will be applied to the input of the frequency disorminator 48,.Fig..2.

Filters BI' and 82T in Fig. 3v comprise a 465'- kilocycle re,i'e 'ztionv filter to prevent any 465= kilocycle modulated componentffrom reaching.

the: control: tube: VT3 and'. causing' anyinstability thereof.

A threshold arrangement embodied in the discriminator 40 precludes the controlled oscillator 42 fromy tuning to any signal below av predetermined minimum level. As thel 46`5`kilocycle component applied to the disc-riminator 40 has substantially a constant level effected by the am.- plier. ilr as hereinbefore mentioned, and as such. level is several decibels abovespurious signals and noise,` the threshold arrangement ensures againstfalse tuning. of the controlled oscillator i2..v -j is so because the voltage produced across the adjustable resistances G8. and 6.9: so biases thenscreen grid of the tube VTI in-Fig. 3 that recticationin the latter cannot commence untilsuch biasing .voltage is overcome by a voltage equivalentv at least to the level of a proper signal.. Asthe level of spurious signals and noise is below that of a proper signal, it is obvious that the controlled oscillator 42 will respond only'to the voltage of proper signals.

It is to be understood that the voltage applied to the input of tube VTI need not be derived exclusively from a'pe'zocrystal and further may b'e a direct current voltage .as well as an alternating current voltage. For example, lsuch voltage may be derived from 'phaseor level'sensit'ive' ap"- paratus, or the outputl of a bridge network or potentiometer circuit; In addition,y acoustic,

electromagnetic, photoel'ectric orv radio pick-up' devi'cesmay also be utilized to supply such voltage. Furthermore; the voltage across the sweeping capacity 12 'is not' necessarily limited to the control of an` oscillator but wtihthe addi"- tonvof suitablieinterr'nediary apparatus may servey tobalance either a bridge network or ay poten"- tiometer, to direct a steerable antenna, to control the movement oi" a boat, an airplane, a tank or a torpedo, and to direct the firing' of a gun.- `Accordingly,v tlie fielder' usefulness of' the discrim-i'nator 4l and-.sweep circuit 4I embraces', in general, such'. automatic control arrangements as. requirea supersensitive. control. over a range which. isy toc widev to: be covered. directly by controlV elements. of the: requiredisensitivity; In. the

y The: frequency discriminator 'n is so 'sensitive present illustration, the piezocrystal is extremely sensitive over a range comprising cycles per second and. furnishesA no control above 20 cycles per second. .The hunting action of the sweep capacity 12 ensures that at some instant during each sweep cyclethe input voltage to the tube VTI is within the restricted control range so that f the sensitive control comprising the tube VTI and sweeping capacity 12 may seize control of the controlled oscillator 4,2 in response to a voltage applied to the input of VTI accomplishing at the same time the arresting of the hunting action ofthesweep circuit 4I.

The capacity 13 connected across. one electrode and the anode ofthe gaseous tube VT2 functions (a) lto store up a charge when' the gaseous tube VT2 ycommences to discharge and to maintain such discharge for a slightly longer interval of time which means that the voltage applied vto the grid of VT3 is held at its lowestvalue for a slightly longer interval of time, and (b). being imperfect and having 'alfinite conductance to take a charge which is higher than that normally rez quired to break down the gaseous tube VT2. Essentially, this has the effect of reducing the breakdown value of the gaseous tube VT2. In this illustration, the breakdown value of Vthe gaseous tube VT2 is made substantially 110 volts and the extinction voltage about 60 volts. provides a 50-volt differential which is more than adequate to sweep the controlled oscillator 42 overy its 475 to 615-kilocycle range of alternating current Waves while, at the same time, allowing sufficient B battery supply to effectthe operation thereof.

Fig. 7 shows the wave form of the sweep voltage produced by the charging and discharging of the sweep capacity 12.r It is saw-tooth in form, sweeping the output of the controlled oscillator 42 over its 475 to 615-kilocycle range of frequencies, Fig. 5, at a uniform rate as the capacity 12 is having a frequency of 465 kil'ocycles, which component actuates the frequency discriminator 4U initially to arrest the hunting action of the sweep circuit 4I and thereafter by means of the latter circuit to control the frequency' of the 475 to 615-kilocycle waves produced by the controlled oscillator 42. Any variation in the frequency of this 465-kilocycle heterodyned component due, for example, to a change in the frequency of the 10 to 150kilocycle measuring waves is reflected as a change in the'impedance of the anode-cath- Iode circuit of the frequency discriminator 40 frequency of 465 kilocycles.

and therefore as a change of the charge on the capacity 12, which charge, as previously seen, serves to control the frequency of the waves produced by the controlledv oscillator 42 such that the heterodyned component applied to the frequency discriminator 4I] tends to maintain the In other words, the frequency difference' between the l0 to 150 and 475 to 615-kilocycle waves heterodyned in thev modulator 36 at a given instant is substantially maintained at ,465 kilocycles throughout the 10 to 150kilocycle range of measuring frequencies.

During the same interval of transmission of the 10 to 150kilocycle measuring waves, a portion of the 475 to 6l5kilocycle waves is simultaneously supplied to the modulator 4B embodied in the detector '21 for heterodyning with the 1I)` to 150kilocycle cross-talk being received thereby on the disturbed pair.v The modulator 46 produces a heterodyned component having a frequency of l465 kilocycles, which component is demodulated with 466-.kilocycle waves to effect an audible I-kilocycle component whose variations in amplitude are utilized for automatically and instantaneously representing cross-talk over the 40 stant over such range as being gradually charged until the breakdown voltage of VT2 is attained whereupon the capacity 12 rapidly discharges. The discharge action of the `sweeping capacity 12 is sufficiently rapid to prevent control of the controlled oscillator 42 on the downward frequency sweep. The A-second interval representsv the aforementioned time func. Ation of the capacity 13.

Accordingly, the operation of Fig.u 2 is as follows:

.During an interval of no transmission of the 10 to 150kilocycle range of measuring waves on the.' disurbing pair and therefore' an 'interval of no input of such waves to the Vautomaticfrequency control I9 and obviously no input to the frequency discriminator 4U, the sweep current 4I is arranged to actuate the controlled oscillator 42 such that the latter produces cyclically a 475 to 615-kilocycle range of alternating current waves. Under this. condition no cross-talk n is present in the disturbed pair and hence no crosstalk is supplied to the heterodyne detector 21.`

Consequently, the automatic frequency control I9 exerts no influence on the heterodyne de-y tector 21. f

During an interval of transmission of the 10 to l 150kilocycle range of measuring waves on the disturbing pair andA consequentlyduring an interval of application of such waves to the automatic frequency control I9, the heterodyning of the 10 to 150kilocycle measuring waves and a portion of the 475 to 615-kilocycle waves in the modulator 36 produces aheterodyne component 10 to 150kilocycle range of measuring waves and Whose frequency is maintained substantially conpointed out above in connection with Fig. 1. The time interval required to complete such measurement is approximately 40 seconds. In other words, thel automatic frequency control I9 serves to supply the 475 to 615-.kilocycle range ofy alternating current waves to the heterodyne detector 21 such that at each instant-'during the transmission of the 10 to 150kilocycle range of measuring waves the frequencyv difference between the latter and the former'waves is 465 kilocycles. Thus, the automatic frequency control I9 serves to tune automatically the heterodyne detector 21 to the crosstalk waves of varying frequency such that at each instant such` cross-talk is represented by a component having a constant frequency and a corresponding amplitude. The audible l-kllocycle component is particularly useful where observations are to be made with telephone receivers. However, it is understood that the amplitude variations of the 465-kilocycle component effected in the `heterodyne detector 21 may also' be readily utilized inv the recorder 28 and indicator'29 to represent cross-talk by tuning the amplifier 55, Fig. 2, to the frequency of such component.

lFig. 8 shows an alternate arrangement forv controlling the action of the sweep circuit 4I which action is identical with vthat described `above in connection with Fig. 3 except in the respect, that a tuned circuit 84 is connected to the control gap C-B of the gaseous discharge tube VT2 and to the output of the oscillatorl tube VT4 by a lead B5. The breakdown potential of the vgap A--B of the gaseous tube VT2 is nor.- mally volts but when a potential of 70 volts is :applied across the control 4gap C-,B, the main q'uate on positive peaks ,to break down the ccntrol gap C-B, and hence the main rgap A-B. The latter in the breakdown vcondition enables the capacity 12 to discharge therethrough until the point F in Fig, 9 is attained,.which point corresponds to the main gap A-B sustaining potential of '70 volts. constitutes the hunting action of Fig. 8 and the latter action continues until a signal impressed onthe discrimina'tor 40 takes control lin the man- `ner set forth above concerning Fig. 3.

Although the invention is particularly described with reference to an automatic measurement of far-end cross-talk, it is not necessarily limited thereto and may be used with equal facility to measure automatically near-end crosstalk, noise, 'transmission and impedance and return loss, in the latter `case a portable bridge is also required. Further, it is to be understood that the invention may be readily used in accomplishing the above measurements in coaxial and square-four cables. ,i The illustrated apparatus is normally operated from a regulated power supply which may beA either `50 or 60 cycles when such is available. In locations too remote from readily accessible comercial power supply, a portable gasoline engine-generator is adapted to furnish the required power. For eld service, apparatus according to the invention, including a suitable gasoline engine-generator, is mounted on a truck or trailer, or the measuring apparatus alone may be packed in specially kdesigned trunks yand shipped to various geographical points. In view of the fact that cross-talk curves over a desired range can be drawn at least in 40 seconds, `as previously pointed out, the same apparatus may be widely used over an extensive geographical area. What is claimed is:

l. In combination, means to produce alternating current waves, means to so control said wave producing means that alternating current waves of a predetermined frequency range are Aproduced cyclically, and means responsive initially to an input voltage to actuateI said `controlling means such that initially lthe cyclic production of said waves is arrested and thereafter respon-4 sive to variations in saidinput voltage to actuate said controlling means such that said Waves are produced in accordance with the variations in said input voltage.

2. In combination, means to produce cyclically a voltage 'of a predeterminedly varying magnitude, and means to actuate said voltage producin g means to arrest the cyclic action thereof and thereafter to control the magnitude of the volt-` age produced thereby. "3. In combination, means comprising a capacity and gaseousl discharge device for producing a voltage of varying magnitude, said capacity cyclically rcharging to the breakdown voltage of said device and after breakdown `discharging therethrough, and means responsive initially to an `input voltage' to limit the charge 'on said capacity toa :magnitude less than that .required- Repetition of this cycle` to discharge said device thereby arresting the cyclic action of the voltage producing means and responsive thereafter to variations in said input voltage to controlthe charge on said capacity and thereby variations in the magnitude of the voltage produced by the voltage producing means:

.4. .In combination, means to produce 'cyclically a voltage of varying magnitude, said means comprising .a three-electrode gaseous discharge device and a capacity connected across the anode and one control yelectrode thereof so .that said capacity is charged to the breakdown voltage .of vsaid device and after breakdown discharged therethrough, and means to control the variations in the`.magnitude of the voltage produced by said. ,producing means, said controlling means comprising a thermionic device which has its plate circuit connected in yparallel with said capacity such that when no signal is applied to the input thereof fthe plate 'circuit has relatively high impedance and exerts no influence on the action of Isaid capacity and when a signal is applied to the input thereof the plate circuit has 'a decreased impedance to divert therethrough a portion of the charge on said capacity thereby :arresting the cyclic ation of said capacity and controlling the charge thereon.

-5. Elin combination, `means to produce alternating current waves, means to control said wave producing means such that alternating current Waves of predetermined frequency range I are cyclically produced, said controlling Imeans comprising a three-electrode gaseous discharge device, a capacity-connected` across the anode andy one control electrode, a tuned circuit .connected to the other'control electrode, a source of biasing voltage applied through said tuned circuit to said second -control electrode and circuit means to connect the output of said wave producing means to said other electrode and tuned circuit, said controlling means arranged such that said capacity is charged at a uniform rate to actuate said Wave -producing means to produce the predetermined range of, alternating current waves which waves serve to build up a voltage across said tuned circuit until at the upper end of the predetermined frequency range Asuch voltage causesa discharge across both control electrodes of the gaseous device whereupon `discharge is instituted across the anode andthe one control electrode to discharge said capacity through said ,said'capacity is limited toa magnitude less than that required to institute discharge across the anode and one ycontrol electrode of said gaseous device thereby arresting the cyclic action of the controlling means and thereafter responsive to variations in the frequency of the input voltage to actuate the controlling means such that the charge on said capacity effects corresponding variations in the frequency ofthe produced Waves.

-6. The method of rdetermining cross-talk between two conductor pairs Which comprises rapplying to one pair alternating current Waves whose frequency varies over a range withinwhich cross-talk is to be determined, deriving from the other lpair cross-talk waves resulting from the transmissionof said alternating current waves over said one pair, automatically translating said cross-.talk waves, under control of said alternating ycurrent waves, from lvarying frequency 'Waves each instant cross-talk at a single frequency, and determining cross-talk from said constant frequency wave.

7. 'I'he method of determining cross-talk between two condluctor pairs which comprises applying alternating current waves of continuously tude corresponding to the cross-talk at each fret quency within the frequency range of the crosstalk to be determined, and determining crosstalk represented by said constant frequency com ponent.

8. The method of observing cross-talk between two conductor pairs which comprises applying to the near end of a first pair alternating current waves whose frequency varies over a range Within which cross-talk is to be observed, generating at the far end of said first pair other alternating current waves whose frequency varies over another range, utilizing both the alternating current waves received at the far end of said rst pair and a portion of said other alternating current waves to control the generation of said latter waves such that a constant'l frequency difference is effected between both said waves over the frequency ranges of both thereof, deriving from the second pair at the far end thereof cross-talk re' sulting from the transmission of said alternating current waves over said first pair, utilizing said cross-talk and another portion of said other alternating current waves to effect automatically a constant frequency component representing at each instant the cross-talk at each frequency throughout the frequency range Within which the cross-talk is to be observed, and observing the cross-talk represented by said constant frequency component. o

9. The method of measuring cross-talk between two conductor pairs which comprises applying alternating current waves of continuously varying frequency to the near end of a first pair, producing at the far end of said first pair other alternating current waves of continuously varying frequency, deriving at the far end of said first pair from both the alternating current waves received thereat and a portion of said other alternating current waves a certain component; which has a tendency to vary in frequency, utilizing'said first-mentioned component to control the production of said other waves such that a constant frequency difference is maintained between both said waves received and produced at the far end of said first pair over the frequency ranges of both said waves, deriving at the far end of the second pair from cross-talk therein and another portion of said other alternating current waves' a constant frequency component having an arnplitude correponding to said cross-talk at each frequency of the range wthin which the crosstalk is to be measured, and observing said crosstalk represented by said second-mentioned component.

10. In combination with two conductor pairs, means to apply to one pair alternating current waves whose frequency varies over a range Within which cross-talk indications are to be made, means to derive cross-talk from the other pair, means responsive to said alternating current waves and said cross-talk to effect automatically `an alternating current wave having a constant frequency and whose amplitude corresponds to said cross-talk at each frequency of the range` within which the cross-talk indications are to be.

utilize said alternating current Waves to effect other alternating current waves whose frequency varies over another range but has a constant frequency difference therefrom, means to derive from both cross-talk in the other pair and said other alternating current waves'a'constant fre-A quency Wave representing at each instant crosstalk at a single frequency over the frequency range within which the cross-talk is to be determined, and means to determine the cross-talk represented by said constant frequency wave.

12. In combination with two conductor pairs, means to apply alternating current; waves of con-` tinuously varying frequency to the near end of a first pair, means at the far end of said first pair tov generate other alternating current Waves of continuously varying frequency, means connected to the far end of said first pair and said other wave generating means and responsive to frequency variations of the waves transmitted on said rst pair and a portion of said other Waves to control said other wave generating means such that a constant frequency difference is maintained between both said waves transmitted on said first pair and said other waves over the fres first pair, means at the far ends of both said'.k

pairs to generate other alternating current waves of continuously varying frequency, means to derive a certain component from a portion of said other waves and cross-talk at the far end of the second pair, means to determine cross-talk rep-y v resented by said certain component, and means to control said other Wave generating means in response to the Waves received at the far endof said first pair and-another portion of said other waves such that a constant frequency difference is maintained between said waves received at the far end of said firstpair and said other waves throughout the frequency ranges of both thereof and thereby to provide said certain component with a constant frequency to represent at each instant the .cross-talk at a single frequency over the frequency range of the cross-talk to be determined, said controlling means comprising means Afor effectively applying a continuously varying actuating voltage to said other Wave generating means in response tothek continuously varying ,frequencies of both said other waves and saidwaves received at the far end of said first 14. The combinatin according -to claim 13 in, which said controlling means embodies a threshold'- device that precludes spurious'voltages from affecting the lvoltage applied to said other wave generating means, 'said threshold device comprising-means for producing a biasing voltage whose magnitude" is atleast of the order of magnitude ofthe voltage of said waves applied to the near endof said first pair. I

15. The combination according to claim 13 in which said controlling means comprises means to sweep said other wave generating means cyclically over its range of frequencies when no alternating current waves are being applied to the near end ofA said first pair, and means responsive initially to a voltage due to both said other waves and said waves received at the far end of said first pair to actuate said sweeping means to arrest the cyclic action of said other wave generating means and responsive thereafter tovariations of such voltage to actuate varyingly said sweeping means and thereby varyingly said other wave generating means.

v.116. The combination according to claim 13 in which said controlling means comprises means to'sweep said other wave generating means cyclically over its range of frequencies when no alternating current waves are being applied to the near end of said first pair, means to derive from both said waves received at the far end of said r'stpair and said other waves a certain component having a tendency to change in frequency in response to the continuously varying frequencies of vboth said waves, and means responsive initially to said last-mentioned certain comlponent to actuate said sweeping means to arrest the cyclic action of said other wave generating means and responsive thereafterto the frequency variations of said last-mentioned certain component to apply a varying voltage to said sweepingmeans and thereby a varying voltage to said other wave generating means.

' 17. A n automatic frequency control system comprising, means including a discriminator network the output of which depends upon the frequency of a signal input thereto over a givenfrequency, control means responsive to said output for maintaining the frequency of said signal input approximately at a predetermined frequency range within said given frequency range when the frequency of said input wave falls within said given frequency range, additional means comprising a searching potential circuit for varying periodically the frequency of said signal input at a frequency lower than any frequency within said given frequency range and over a frequency range which is` wider than and which overlaps said given frequency range, and means responsive to the output of said discriminator for rendering said additional means ineffective when the resulting frequency of said signal input falls within said given frequency range.

18. In a carrier-wave signal receiver of the superheterodyne type. an automatic frequency control system comprising, a discriminator network, an oscillator control tube, means including said discriminator network for developing a frequency-control bias and applying it to saidtube, additional means for effectively applying a lowfrequency alternating potential to saidcontrol tube for varying periodically the oscillator frei quency, whereby automatically to tune the receiver to signals which initially are out of the frequency-response range of the discriminator, and means responsive to the resulting frequency of the signal input to said network for removing said low-frequency voltage from said control tube whensaid resulting frequency has any value within the frequency-responserange o f'said discriminator.

19. In a carrier-wave signal receiver of the superheterodyne type, an automatic frequency control system comprising, anexceedingly sharp discriminator network the output of which varies in response to variation of the frequency of an intermediate-carrier signal input thereto overv a given frequency range, an oscillator control tube, means responsive to said output for controlling said controly tube to adjusti the frequency of said intermediate-carrier signal input to a frequency within a desired small frequency range when said frequency of said intermediate-carrier signal input has any value within said given frequency range, additional means for applying a low-frequency alternating potential to said oscillator control tube for varying periodically the oscillator frequency' so that it sweeps at a frequency lower than the intermediate-carrier frequency of said receiver and over a frequency range which overlaps and is wider than said given frequency range, and means responsive t0 the output of said discriminator network for rendering said additional means inoperative when the resulting frequency of said intermediate-carrier signal input falls within said given frequency range.

20. In a carrier-wave signal receiver of the superheterodyne type, an automatic frequency control system comprising, a discriminator network, an oscillator control tube, means including said discriminator network for developing a frequencycontrol bias and applying it to. said tube, additional means for effectively applying a cyclically varying potential to said control tube for varying periodicallythe oscillator frequency, whereby automatically to tune the receiver to signals which initially are out of the frequency-response range of the discriminator, and means responsive to the resulting frequency of the signal input to said network for removing said cyclically varying' voltage from said control tube when said resulting frequency has any value within the frequencyresponse range of said discriminator. v

21. In a carrier-wave signal receiver of the superheterodyne type, an automatic frequency control system comprising, an exceedingly sharp discriminator network the output of which Varies in response to variation of the frequency of an intermediate-carrier signal input thereto overl a given frequency range, an oscillator control tube, means responsive to said output for controlling said control tube to adjust the frequency of said intermediate-carrier signal input to a frequency within a desired small frequency range when said frequency of said intermediate-carrier signal input has any value within said given frequency range, additional means for applying a cyclically varying potential to-said oscillator control tube for varying periodically the oscillator frequency so that it sweeps at a frequency lower than the intermediate-carrier frequency of said receiver and over a frequency range which overlaps and is wider than said given frequency range, and means responsive to the output of said discriminator A network forl rendering said additional means incies at least as Wide as that consisting of a plurality lof adjacent communication channels, means free of mechanical inertia connected to said reactance generating device and arranged to automatically generate oscillations of loW frequency and of suflicient amplitude to repeatedly change the reactance generating device by an amount sufficient to cause the tuning o'f said circuit throughout said `Wide range of frequencies, and frequency responsive means responsive to the vreception of a Wave of any frequency Within said range for maintaining the tuning of said circuit approximately at a desired frequencyfwithin said range of frequencies.

23. A receiver' comprising an amplifier having an input circuit having an acceptance band wide enough to accommodate the frequency range employed in broad band carrier Wave transmission, a tunable circuit, means free of mechanical inertia arranged to automatically tune said tunable circuit over a range of frequencies different from but approximately as wide as the acceptance band of said input circuit, a mixing device connected to the output of said amplifier and to said tunable circuit, and a frequency responsive device connected to the output of said mixing device and arranged to automatically render ineffective the action of said automatic tuning means on the tuning of said tunable circuit in response to the impression on said input circuit of waves having a frequency Within said acceptance band.

241. A receiver comprising -an input circuit having an acceptance band wide enough to accommodate the frequencies used in broad band carrier transmission, a tunable circuit, means free of mechanical inertia for continuously and automatically tuning said tunable circuit over a range of frequencies substantially as wide as the acceptance band of said input circuit, a mixing device connected to said input and tunable v circuits and a frequency responsive device con.- nected to the output of said mixing device and arranged to render ineffective the action of said automatic tuning means on the tuning ofA said tunable circuit 'in response to currents in the output of said mixing device of a predetermined frequency.

25. A receiver of the superheterodyne type tunable to receive currents of different carrier frequencies within a Wide band of frequencies, said receiver comprising an oscillator and an intermediate frequency amplifier, a mixing device connected to the input of said intermediate frequency amplifier and to said oscillator, a circuit network connected to the input of said mixing device and having an admittance band suiciently wide to transmit at any instant waves covering a broad band, means free of mechanical inertia electrically connected to said oscillator and arranged to automatically vary the frequency of the currents generated by said oscillator throughout a range of frequencies substantially as wide as said band, a circuit device coupled to the output circuit of said intermediate frequency amplifier and responsive to the frequency of the currents passing through said amplifier and a reactance generating device connected to said circuit device and arranged upon the energization thereof by said circuit device to maintain the frequencies of the currents generated by said oscillator substantially constant.

l26. In combination with a receiver of the superheterodyne type adapted to receive high fre- I quency currents of different carrier Wave frequencies and comprising a first detector and an oscillator, said oscillator comprising a vacuum i tube having coupled input and output circuits,

said output circuit comprising a coil and a fixed condenser connected in parallel and providing lumped constants determining the resonant frequency, of said output circuit, control means for adjusting the inductance of said coil to cause said oscillator to generate currents of a predetermined frequency, and means free of mechanical inertia for automatically causing the frequency of the oscillator to vary selectively from a first frequency to a second frequency and from the second frequency to said first frequency a plurality of times in succession, said first and second frequencies being adapted to combine with the highest and lowest received carrier frequencies to form a beat frequency.

27. A receiving system comprising in combination, an input circuit having an acceptance band wide enough to accommodate the frequencies used in broad band carrier transmission, a communication channel and automatic means for cyclically varying the frequency of the input energy impressed on said communication channel by currents in said input circuit having any frequency Within said band at a substantially steady rate in one direction and at a rate much more abrupt in the opposite direction.

EDWIN P. FELCH, JR. 

